Autosomal dominant polycystic kidney disease (ADPKD) is a common inherited disorder that progresses to end-stage renal disease in approximately 50% of affected individuals. Studies using positional cloning have identified two genes, PKD1 and PKD2, that are responsible for most cases of human ADPKD. The PKD1 and PKD2 genes encode large membrane proteins, named polycystin-1 and polycystin-2, that are of unknown function. Genes that are homologous to PKD1 and PKD2 have also been identified in the mouse genome. Targeted disruption of the mouse Pkd1 and Pkd2 genes causes severe renal developmental abnormalities, demonstrating that the genes are essential for normal kidney organogenesis. The expression of polycystin-2 and polycystin-1 in the developing kidney is differentiation-specific, likely reflecting developmental regulation of Pkd2 and Pkd1 gene expression. The proposed studies will test the hypothesis that Pkd2 and Pkd1 are developmentally regulated genes that are essential for renal tubular epithelial differentiation during kidney organogenesis The first aim is to determine the mechanisms of developmental regulation of polycystin gene expression. Norther blot analysis and in situ hybridization will be performed to define the spatiotemporal patterns of expression of Pkd2 and Pkd1 transcripts in the developing kidney. The expression of green fluorescent protein (GFP) will be examined in mice carrying a modified Pkd2 BAC transgene. Reporter gene assays will be performed in transfected cells and transgenic mice to identify regions of the Pkd2 promoter that mediate differentiation-specific regulation. The second aim is to define the specific abnormalities of kidney development in Pkd2 mutant mice. The effects of the mutations on nephron induction, branching morphogenesis, glomerulogenesis, and tubular differentiation will be examined using specific markers. An in vitro organ culture model that recapitulates the developmental abnormalities will be created. The model will be used to test whether inactivation of Pkd2 arrests tubular maturation and to identify the fate of polycystin-deficient cells. Collectively, these studies will provide novel sights into the regulation of polycystin-deficient cells. Collectively, these studies will provide novel insights into the regulation of polycystin gene expression and the roles of polycystins in kidney development.